1. Field of the Invention
The present invention relates to a process for controlling the level of an ammonium fluoride-containing acid solution in an acid vessel, and more particularly to a process for prolonging the period of time before the dummy wet station is process abnormal stop due to the accumulation of ammonium fluoride crystals on the high level sensing tube by providing at least one hole in the sidewall of the high level sensing tube.
2. Description of the Prior Art
Nowadays, wafer cleaning is often conducted in a tool called dummy wet station (DWS). A batch of wafers is introduced in an acid vessel of the DWS for cleaning. Then, another batch of wafers is introduced in the acid vessel for cleaning. After many batches of cleaning, the acid solution in the acid vessel becomes dirty and needs to be replaced. A commonly used acid solution for cleaning is buffered HF (BHF), which includes hydrofluoric acid (HF) and ammonium fluoride (NH.sub.4 F).
During the process of adding the acid solution to the acid vessel, a low level sensor and a high level sensor are generally used to detect the level of the acid solution so as to control the acid solution level to reach a predetermined value. Referring to FIG. 1, the bottom portion of the tube 1 of the low level sensor is immersed in the acid solution 30 received in a vessel, while the bottom portion of the tube 2 of the high level sensor is disposed above the liquid surface of the acid solution. When the low level sensor detects that the liquid surface is too low, it will output a signal for introducing the acid solution into the vessel; thus, the process of introducing the acid solution proceeds. When the high level sensor detects that the liquid surface is too high, it will output a signal for draining the acid solution; thus, the acid solution is allow to flow out until the level of the acid solution reaches a predetermined value.
The principle of the high level sensor is described below. Nitrogen gas is introduced into the tube 2 of the high level sensor, hereinafter referred to as high level sensing tube, and the nitrogen pressure is detected by a CPU (central processing unit). Referring to FIG. 1, when the acid solution level has not reached the bottom A of the high level sensing tube 2, nitrogen can flow through the bottom A of the high level sensing tube and the nitrogen pressure is detected to as P.sub.1. Referring to FIG. 2, when the acid solution level reaches the bottom A of the high level sensing tube 2, nitrogen can not easily flow through the bottom A of the high level sensing tube and the nitrogen pressure is detected to as P.sub.2, which is certainly larger than P.sub.1. When the CPU receives a signal that the nitrogen pressure is P.sub.2, it indicates that the acid solution level has reached a high level. That is to say, no more acid solution should be added. Therefore, the CPU will output a signal for draining the acid solution until the nitrogen pressure received by the CPU is P.sub.1. In this manner, the level of the acid solution can be controlled to a predetermined value; thus, the acid solution will not overflow.
When wafers are cleaned in the BHF solution, some ammonium fluoride contained in the BHF solution will crystallize on the wall of the high level sensing tube, thus jamming the tube. Referring to FIG. 3, when the acid solution level has not reached the bottom A of the high level sensing tube, but the bottom A is jammed with the ammonium fluoride crystals 40, nitrogen gas can not easily flow through the bottom A; therefore, the nitrogen pressure detected is P.sub.2. As mentioned above, once the CPU receive the signal that the nitrogen pressure is P.sub.2, it will output a signal for draining the acid solution until the nitrogen pressure received by the CPU is P.sub.1. However, even if some acid solution is drained, the nitrogen pressure is still P.sub.2 and can not become P.sub.1, since the bottom A of the high level sensing tube has been jammed. Thus, the CPU will continually output the draining signal, and the acid solution will eventually become empty. When the acid solution is drained, the DWS tool should be totally shut down and can not operate, and the process is interrupted. This increases the frequency of preventive maintenance (PM) and also causes waste of the acid solution, thus decreasing production efficiency.